Nitrogen-bonded substituents hybridization state

At the halfway point along the path to the inverted pyramid, the transition state is reached. Here the molecule is planar, with 120° R—N—R bond angles. At this point, the hybridization of the central nitrogen is s The inversion process maintains a mirror plane of symmetry. In the alkanes, we could certainly imagine the beginning of the flattening process, but there is no way to force the fourth substituent through the molecule to the other side (Fig. 6.40). 

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